The present invention relates to a pelletizing die for a synthetic resin extruding machine.
Pelletizing dies are widely used for shaping the primary raw materials in the synthetic macromolecule material manufacturing industry, and serve to manufacture pellets of synthetic resin which are used in molding.
Since the primary raw material is generally in the shape of powder or in the melted state, the raw material is usually mixed and kneaded, caused to pass through a die to be formed into a rope-like shape, and then cut by a cutter blade rotating at a high speed to thereby obtain pelletized material. However, because the rope-like synthetic resin extruded out of the nozzle is low in viscosity, the rope-like synthetic material cannot be properly cut off, so that each pellet may leave a tail which distorts the pellet's shape and thereby reduces the commercial value of the pellets. This can result when the force urging the cutter blade against the die is weak or when the parallelism between the die surface and the cutter blade is not good. If the cutter blade is strongly urged against the die to solve this problem, the die surface will be quickly worn out. For this reason, in the prior art, a sintered hard tip 1 or 2 made of a sintered hard alloy and formed into a cylindrical shape or into a rectangular tile-like shape, as shown in FIG. 1 or in FIG. 2 respectively, is embedded and fixed by soldering in a die body 4 of a pelletizing die 3. These structures are shown in FIGS. 4 and 5, which are partial views taken along the line IV--IV of FIG. 3 which is a plan view of a pelletizing die. In an arrangement, a sintered hard tip 1 or 2 is not produced in advance and a sintered hard alloy such as a tungsten carbide alloy is metal-sprayed onto a die surface to form a hardened layer corresponding to the sintered hard tip 1 or 2.
The thus arranged conventional pelletizing die is disadvantageous in that the sintered hard tip 1 or 2 or the hardened layer may come off after an operation time of about one year, because the coefficient of linear expansion of the die body is largely different from that of the sintered hard tip 1 or 2 or the hardened layer due to the difference in their material quality. Therefore, the soldered portion is apt to be cracked due to the temperature changes and the electrical corrosion which occurs at the soldered portion due to warm water, and the sintered hard tip 1 or 2 or the hardened layer is subjected to shocks due to the high speed rotation of the cutter blade, so that the adhesive force at the soldered portion is weakened to cause a crack.